In manufacturing automotive interior panels that include air bag deployment portions, it is common practice to use vinyl, typically polyvinyl chloride (PVC), to form a thin soft pliable outer shell or skin for the panel. The skin material may also include the use of urethanes (e.g., PU) olefins (e.g., PP, PE, TPO, ETP-TPO), esters (e.g., COPE), styrenes (e.g., ADS, ASA) and rubbers (e.g. TPO, ETP-TPO, ABS) in various compositions. One side of a soft plastic foam layer is then bonded to the skin and the other side of the foam layer is bonded to an injection-molded plastic substrate. The substrate may also be constructed from materials such as metal, plastic composite or wood fiber composite using other forming techniques well known in the art. The skin, including its air bag cover portion, is commonly formed by casting or spraying plastic material against a heated mold surface that defines the desired shape of the appearance side of the panel. Two examples of materials that may be used in casting shells include miniature spherical (0.020") pellets formed by extruding plastic through a very small die, and a powder formed by using a high intensity mixer to diffuse plasticizer and colorant into PVC resin particles to yield a particle size averaging 300 microns. The mold is commonly referred to as a shell tool and typically has an electroformed nickel surface that produces a grained surface or other desired texture on the outer side of the skin for styling purposes.
It is also common practice to provide a tear seam in the air bag cover portion of the skin that air bag inflation force tears open to allow the air bag to deploy. The tear seam may be formed in various ways in the air bag cover skin such as with a visible weakening groove in the outer side, a hidden weakening groove in the back side or with a tear seam strip of relatively weak plastic material that is incorporated in the cover skin and may be hidden from view by painting the air bag cover. The opening for the air bag in the cover skin may also be formed by a cutting device located behind the skin. The substrate also includes suitable means, such as a hinged door arrangement; to create an opening in the substrate corresponding to the opening created in the cover skin. Upon air bag deployment a portion of the skin and a portion of the foam layer become the outer and intermediate layers, respectively, of such a door or doors and swing outward with the door or doors.
Certain polyvinyl chloride (PVC) formulations such as Drysol compounds DL1011 and DL1013 are well suited, in durability, fade and heat resistance, to meeting the general requirements of an instrument panel skin which is subject to sun rays passing through a proprietary formulations of PVC resins, plasticizers, stabilizers and pigments. Their formulations may include, but are not limited to the following: 50 to 60% PVC resin by weight, 30 to 45% plasticizer by weight, and 0.1 to 5% pigment by weight. On the other hand, the air bag cover requirements further include the ability to remain ductile at very low temperatures to prevent the cover from fragmenting when an inflating air bag impacts it. Because of cold embrittlement, PVC formulations such as DL1011 and DL1013 cannot provide for satisfactory air bag deployment performance. These PVC formulations tend to fragment at the extremely low temperatures that some automobile manufacturers specify. This is particularly true where a tear seam is formed in the skin or a cutter device is used to create an air bag deployment opening. To meet such extreme low temperature ductility requirements, a completely separate air bag door of PVC or other plastic materials may be installed in the instrument panel to obviate the need to tear or cut the PVC skin of the instrument panel to create an opening for the air bag. However, some automobile manufacturers prefer a hidden door. A completely separate air bag door is both difficult to hide and costs more than a skin having an integral tear seam that is either visible or hidden.
Certain PVC formulations such as Drysol compound DL3020, and certain polyurethane (PU) elastomers remain elastic or ductile at temperatures substantially below where PVC formulations such as Drysol compounds DL1011 or DL1013 become brittle or inelastic. However, PU and DL3020 are relatively expensive compared to DL1011 and DL1013. DL3020, for example, costs about 50% more than DL1013. In other words, standard PVC formulations such as Drysol compounds DL1011 and DL1013 are very well suited to general instrument panel skin requirements and are cheaper than PU and DL3020, but are not as ductile as PU or DL3020 at extremely low temperatures. There have been various attempts to cost-effectively meet the different instrument panel and air bag cover requirements with a single skin structure. However, it has proven to be especially difficult to cost-effectively design an instrument panel with satisfactory air bag deployment characteristics at extremely low temperatures.